As a display element provided in a display device, there have conventionally been an electric optical element in which a luminance is controlled by an applied voltage and an electric optical element in which a luminance is controlled by a flowing current. Typical examples of the electric optical element in which a luminance is controlled by an applied voltage include a liquid crystal display element. Typical examples of the electric optical element in which a luminance is controlled by a flowing current include an OLED. The OLED is also called an organic electro luminescence (EL) element. An organic EL display device that uses the OLED being a spontaneous light-emitting electric optical element can be easily reduced in thickness and power consumption and increased in luminance as compared to the liquid crystal display device that requires a backlight, a color filter, and the like. Hence, in recent years, development of the organic EL display device has been actively advanced.
There are two kinds of driving techniques for the organic EL display device, namely, a passive matrix technique (also referred to as simple matrix technique) and an active matrix technique. As for an organic EL display device employing the passive matrix technique (hereinafter referred to as “passive matrix type organic EL display device”), its structure is simple, but a large size and high definition are difficult to achieve. In contrast, as for an organic EL display device employing the active matrix technique (hereinafter referred to as “active matrix type organic EL display device”), a large size and high definition can be easily realized as compared to the passive matrix type organic EL display device.
The active matrix type organic EL display device includes a plurality of pixel circuits arranged in a matrix form. Further, the pixel circuit of the active matrix type organic EL display device typically includes an input transistor for selecting a pixel and a drive transistor for controlling supply of a current to the OLED. Hereinafter, a current that flows from the drive transistor to the OLED is referred to as a “drive current”.
As for the drive transistor, a thin film transistor (TFT) has typically been used. However, regarding the drive transistor, variations in characteristic tend to occur. The variations in characteristic of the drive transistor become a factor of variations in luminance. For example, even when the same gradation signal (gradation voltage) is given to all pixels, a different luminance appears with respect to each of the pixels. It should be noted that the “characteristic of the drive transistor” referred herein is, for example, a threshold voltage, mobility, or the like of the drive transistor.
It is known that degradation of the OLED proceeds as the light emission time becomes longer, resulting in deterioration in light emission luminance from an initial state. That is, when degradation of the OLED proceeds, a desired light emission luminance is not acquired even when the same drive current as in the initial state is allowed to flow. Hereinafter, the degradation of the OLED which proceeds as the light emission time thereof becomes longer is referred to as “time degradation”. Further, for example when the time degradation of the OLED proceeds more at a certain pixel than at peripheral pixels, a luminance difference between those pixels is visually recognized. Such a phenomenon is called “burning”.
It should be noted that the following related art documents are known in connection with the present invention. Japanese Patent Application Laid-Open No. 2001-175221, Japanese Patent Application Laid-Open No. 2007-286295 and Japanese Patent Application Laid-Open No. 2006-284971 each discloses a technique for suppressing occurrence of burning in a display device. Specifically, in a display device described in Japanese Patent Application Laid-Open No. 2001-175221, processing of lowering light emission luminance of pixels to the same level as a light emission luminance of a pixel where burning has occurred (processing of adjusting light emission luminance of other pixels in accordance with a light emission luminance of a degraded pixel) is performed, to make burning inconspicuous. In a display device of Japanese Patent Application Laid-Open No. 2007-286295, a high luminance display screen (all-white screen) is displayed every time a predetermined user's event is detected, to suppress occurrence of a difference in light emission luminance within the display screen. Japanese Patent Application Laid-Open No. 2006-284971 discloses a burning correction apparatus provided with: a correction amount deciding portion for deciding a correction amount corresponding to each pixel based on a degradation amount of each pixel; a variation determining portion for calculating information indicating a degree of variations in correction amount distribution; and a gradation converting portion for converting an input gradation value to an output gradation value with reference to a gamma curve formed so as to compress a gradation difference more effectively as the variations in correction amount distribution is larger.
Further, Japanese Patent Application Laid-Open No. 2006-195313 discloses a technique of reducing a memory capacity required for saving burning information. Japanese Patent Application Laid-Open No. 2012-141626 discloses a technique of suppressing occurrence of luminance unevenness in a display device. Japanese Patent Application Laid-Open No. 2007-279290 discloses a technique of reducing a memory capacity required for saving correction data for suppressing variations in luminance.